1. Field of the Invention
The present invention relates to a polarization rotator for rotating the plane of polarization of linearly polarized light by an arbitrary angle. Such a rotator may be used, for example, as part of a parallax barrier in a display or as an optical modulator in telecommunications. The present invention also relates to parallax barriers, displays and optical modulators including such polarization rotators.
2. Description of the Related Art
A known type of liquid crystal device (LCD) is the twisted nematic (TN) type. In such a device, a layer of nematic liquid crystal material is disposed between opposing alignment surfaces, such as rubbed polyimide, which cause the liquid crystal director to twist through 90° from one alignment surface to the other in the absence of an applied field. In the absence of an applied field, the plane of linearly polarized light passing through the device is rotated by 90°.
A relatively low voltage, for example between 0 and 3 volts, is sufficient to switch such TN LCDs. When switched to the non-rotating state, the mid-plane tilt angle of the liquid crystal director rises above a predetermined angle such that the surface regions of the liquid crystal material adjacent the alignment surfaces may be considered as substantially decoupled voltage-dependent optical retarders. Each of these retarders may be represented by a single optic axis along the alignment direction of the alignment surface. The alignment directions of the opposing surfaces are orthogonal to each other so that, provided the pre-tilt of the liquid crystal molecules at the alignment surfaces are substantially equal, the retardations of the surface regions of the liquid crystal layer effectively cancel each other out so that the device provides substantially zero retardation. However, this self-compensation only occurs for twist angles of 90°.
TN LCDs having twists other than 90° are also known. When linearly polarized light is passed through such devices, the polarization is converted to elliptical polarization. In order to switch such a device so as to have substantially no effect on the polarization of light passing therethrough, a theoretically infinite voltage is required in order to reduce the retardation to zero.
Beynon et al, Journal of the SID, 1999, 7, 71 disclose a technique for calculating linear to circular polarization conversion in a TN LCD and apply the result to reflective displays comprising a liquid crystal spatial light modulator disposed between a reflector and a polariser.